sunxi: power: axp818: Add support for switch SW

The AXP818 has a switchable output, SW. This is commonly used for
controlling power to the LCD backlight.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>